This invention relates to web coating apparatus such as is used, for example, to coat paper for magazines. More specifically, this invention relates to a new type of holder to mount the rotating metering rod in the so-called"rod coater" type coating apparatus.
Rod coaters are well known in the papermaking industry as exemplified by U.S. Pat. Nos. 3,143,438; 3,179,083 and 3,683,851. Typical rod holders used in rod coating apparatus are exemplified by U.S. Pat. Nos. 3,683,851 and 3,701,335.
In existing rod coater equipment, as shown in the above cited patent apparatus, the rod is inserted into the rod holder longitudinally from one end which necessitates a bench assembly since the rod is inserted with a press or interference fit. The rod holder with inserted rod then in turn is mounted longitudinally in the coating apparatus which requires the coater to be "down", or in an inoperative status.
In addition, existing types of rod holders have no capability for having the size of the rod cavity adjusted to accommodate wear during the course of operation. In all rod coaters, the relatively abrasive coating materials containing various types of oxides and carbonates, wear away the surface of the rod holder cavity bearing against the rotating rod. Eventually, the gap between the rod and rod holder becomes so large that lubricating water escapes to deleteriously affect the coating applied to the web. Further, localized differences in the wear of the rod holder caused by variations in the distribution of the coating material which is picked up by the rod and carried into the cavity cause uneven wear which results in more water escaping in those localized places to delute the coating and thereby cause streaking in the web.
In order to extend the life of prior rod holders in rod coaters, the cylindrical cavity, or bore, formed in the rod holder was made smaller than the diameter of the rod, such as, for example, a 0.003 inch interference fit, so that the rod holder could accommodate additional wear before the gap between the rod and rod holder become too large to prevent an excessive amount of water to escape and mark the web being coated. Perhaps because of the interference fits, rods on existing coaterscommonly last only from ten days to two weeks before their chrome plating wears off. In addition, the wear of the cylindrical cavity in the rod holder of prior rod coaters becomes excessive (i.e. lubricating water begins to leak) typically after about two to three weeks of operation at which time the coater must be shut down to replace the rod holder. Naturally, this is very costly both from a standpoint of coater down time as well as the cost of replacing rods and rod holders.
Besides the short life of existing types of rod hodlers, their operation requires high initial torque to turn the rod due to the interference fit with the rod holder. As the rod holder cavity wears and becomes larger, the amount of lubricating water carried on the rod's surface and transferred to the coating increases and this may vary so much from the initial amount of water carried on the rod as to noticeably affect the appearance of the coating compared with its appearance when the rod holder is new.